mseal: add documentation
Add documentation for mseal(). Link: https://lkml.kernel.org/r/20240415163527.626541-5-jeffxu@chromium.org Signed-off-by: Jeff Xu <jeffxu@chromium.org> Reviewed-by: Kees Cook <keescook@chromium.org> Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com> Cc: Dave Hansen <dave.hansen@intel.com> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Cc: Guenter Roeck <groeck@chromium.org> Cc: Jann Horn <jannh@google.com> Cc: Jeff Xu <jeffxu@google.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Jorge Lucangeli Obes <jorgelo@chromium.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Matthew Wilcox (Oracle) <willy@infradead.org> Cc: Muhammad Usama Anjum <usama.anjum@collabora.com> Cc: Pedro Falcato <pedro.falcato@gmail.com> Cc: Stephen Röttger <sroettger@google.com> Cc: Suren Baghdasaryan <surenb@google.com> Cc: Amer Al Shanawany <amer.shanawany@gmail.com> Cc: Javier Carrasco <javier.carrasco.cruz@gmail.com> Cc: Shuah Khan <shuah@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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Jeff Xu
Andrew Morton
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@ -20,6 +20,7 @@ System calls
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futex2
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ebpf/index
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ioctl/index
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mseal
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Security-related interfaces
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===========================
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@ -0,0 +1,199 @@
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.. SPDX-License-Identifier: GPL-2.0
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=====================
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Introduction of mseal
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=====================
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:Author: Jeff Xu <jeffxu@chromium.org>
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Modern CPUs support memory permissions such as RW and NX bits. The memory
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permission feature improves security stance on memory corruption bugs, i.e.
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the attacker can’t just write to arbitrary memory and point the code to it,
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the memory has to be marked with X bit, or else an exception will happen.
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Memory sealing additionally protects the mapping itself against
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modifications. This is useful to mitigate memory corruption issues where a
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corrupted pointer is passed to a memory management system. For example,
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such an attacker primitive can break control-flow integrity guarantees
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since read-only memory that is supposed to be trusted can become writable
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or .text pages can get remapped. Memory sealing can automatically be
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applied by the runtime loader to seal .text and .rodata pages and
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applications can additionally seal security critical data at runtime.
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A similar feature already exists in the XNU kernel with the
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VM_FLAGS_PERMANENT flag [1] and on OpenBSD with the mimmutable syscall [2].
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User API
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========
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mseal()
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-----------
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The mseal() syscall has the following signature:
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``int mseal(void addr, size_t len, unsigned long flags)``
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**addr/len**: virtual memory address range.
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The address range set by ``addr``/``len`` must meet:
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- The start address must be in an allocated VMA.
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- The start address must be page aligned.
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- The end address (``addr`` + ``len``) must be in an allocated VMA.
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- no gap (unallocated memory) between start and end address.
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The ``len`` will be paged aligned implicitly by the kernel.
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**flags**: reserved for future use.
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**return values**:
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- ``0``: Success.
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- ``-EINVAL``:
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- Invalid input ``flags``.
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- The start address (``addr``) is not page aligned.
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- Address range (``addr`` + ``len``) overflow.
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- ``-ENOMEM``:
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- The start address (``addr``) is not allocated.
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- The end address (``addr`` + ``len``) is not allocated.
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- A gap (unallocated memory) between start and end address.
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- ``-EPERM``:
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- sealing is supported only on 64-bit CPUs, 32-bit is not supported.
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- For above error cases, users can expect the given memory range is
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unmodified, i.e. no partial update.
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- There might be other internal errors/cases not listed here, e.g.
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error during merging/splitting VMAs, or the process reaching the max
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number of supported VMAs. In those cases, partial updates to the given
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memory range could happen. However, those cases should be rare.
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**Blocked operations after sealing**:
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Unmapping, moving to another location, and shrinking the size,
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via munmap() and mremap(), can leave an empty space, therefore
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can be replaced with a VMA with a new set of attributes.
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Moving or expanding a different VMA into the current location,
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via mremap().
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Modifying a VMA via mmap(MAP_FIXED).
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Size expansion, via mremap(), does not appear to pose any
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specific risks to sealed VMAs. It is included anyway because
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the use case is unclear. In any case, users can rely on
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merging to expand a sealed VMA.
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mprotect() and pkey_mprotect().
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Some destructive madvice() behaviors (e.g. MADV_DONTNEED)
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for anonymous memory, when users don't have write permission to the
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memory. Those behaviors can alter region contents by discarding pages,
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effectively a memset(0) for anonymous memory.
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Kernel will return -EPERM for blocked operations.
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For blocked operations, one can expect the given address is unmodified,
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i.e. no partial update. Note, this is different from existing mm
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system call behaviors, where partial updates are made till an error is
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found and returned to userspace. To give an example:
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Assume following code sequence:
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- ptr = mmap(null, 8192, PROT_NONE);
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- munmap(ptr + 4096, 4096);
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- ret1 = mprotect(ptr, 8192, PROT_READ);
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- mseal(ptr, 4096);
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- ret2 = mprotect(ptr, 8192, PROT_NONE);
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ret1 will be -ENOMEM, the page from ptr is updated to PROT_READ.
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ret2 will be -EPERM, the page remains to be PROT_READ.
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**Note**:
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- mseal() only works on 64-bit CPUs, not 32-bit CPU.
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- users can call mseal() multiple times, mseal() on an already sealed memory
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is a no-action (not error).
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- munseal() is not supported.
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Use cases:
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==========
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- glibc:
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The dynamic linker, during loading ELF executables, can apply sealing to
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non-writable memory segments.
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- Chrome browser: protect some security sensitive data-structures.
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Notes on which memory to seal:
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==============================
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It might be important to note that sealing changes the lifetime of a mapping,
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i.e. the sealed mapping won’t be unmapped till the process terminates or the
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exec system call is invoked. Applications can apply sealing to any virtual
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memory region from userspace, but it is crucial to thoroughly analyze the
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mapping's lifetime prior to apply the sealing.
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For example:
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- aio/shm
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aio/shm can call mmap()/munmap() on behalf of userspace, e.g. ksys_shmdt() in
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shm.c. The lifetime of those mapping are not tied to the lifetime of the
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process. If those memories are sealed from userspace, then munmap() will fail,
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causing leaks in VMA address space during the lifetime of the process.
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- Brk (heap)
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Currently, userspace applications can seal parts of the heap by calling
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malloc() and mseal().
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let's assume following calls from user space:
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- ptr = malloc(size);
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- mprotect(ptr, size, RO);
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- mseal(ptr, size);
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- free(ptr);
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Technically, before mseal() is added, the user can change the protection of
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the heap by calling mprotect(RO). As long as the user changes the protection
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back to RW before free(), the memory range can be reused.
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Adding mseal() into the picture, however, the heap is then sealed partially,
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the user can still free it, but the memory remains to be RO. If the address
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is re-used by the heap manager for another malloc, the process might crash
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soon after. Therefore, it is important not to apply sealing to any memory
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that might get recycled.
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Furthermore, even if the application never calls the free() for the ptr,
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the heap manager may invoke the brk system call to shrink the size of the
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heap. In the kernel, the brk-shrink will call munmap(). Consequently,
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depending on the location of the ptr, the outcome of brk-shrink is
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nondeterministic.
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Additional notes:
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=================
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As Jann Horn pointed out in [3], there are still a few ways to write
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to RO memory, which is, in a way, by design. Those cases are not covered
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by mseal(). If applications want to block such cases, sandbox tools (such as
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seccomp, LSM, etc) might be considered.
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Those cases are:
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- Write to read-only memory through /proc/self/mem interface.
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- Write to read-only memory through ptrace (such as PTRACE_POKETEXT).
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- userfaultfd.
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The idea that inspired this patch comes from Stephen Röttger’s work in V8
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CFI [4]. Chrome browser in ChromeOS will be the first user of this API.
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Reference:
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==========
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[1] https://github.com/apple-oss-distributions/xnu/blob/1031c584a5e37aff177559b9f69dbd3c8c3fd30a/osfmk/mach/vm_statistics.h#L274
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[2] https://man.openbsd.org/mimmutable.2
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[3] https://lore.kernel.org/lkml/CAG48ez3ShUYey+ZAFsU2i1RpQn0a5eOs2hzQ426FkcgnfUGLvA@mail.gmail.com
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[4] https://docs.google.com/document/d/1O2jwK4dxI3nRcOJuPYkonhTkNQfbmwdvxQMyXgeaRHo/edit#heading=h.bvaojj9fu6hc
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